It is common practice to disperse spacers such as spherical silica beads several micrometers in diameter between substrates in order to maintain the spacing between the substrates inside the liquid crystal cell in a liquid crystal electro-optical device. Where spacers are used, a decrease in the substrate spacing can be prevented but an increase in the spacing cannot be prevented. As the area of display screen is increased, the substrates are distorted or the distribution of the liquid crystal material becomes more nonuniform. These spacers can by no means prevent these undesirable situations.
A countermeasure taken against this is to use a method consisting of adding adhesive particles to the spacers and dispersing these particles in the cell so as to maintain the substrate spacing. In this method, however, the orientation around the adhesive tends to be disturbed.
A method of solving the problem with the method of maintaining the substrate spacing has been proposed by the present inventors and others as disclosed in Japanese Patent application Ser. No. 55237/1993. In particular, the substrate spacing is maintained by polymerized column spacers which consist of resinous columns precipitated (deposited) out of a liquid crystal material. More specifically, a mixture of the liquid crystal material and an uncured resin is inserted between substrates. The molecules of the liquid crystal material are oriented. The resinous material is precipitated (deposited) and cured to form resinous columns. Both substrates are bonded together with these columns.
In this method, the resinous columns can be cured while the liquid crystal molecules are oriented inside the cell. The liquid crystal is precipitated (deposited) out of the mixture of the liquid crystal material and the uncured resin, and the molecules of the liquid crystal are oriented. As a result, the resin is shaped. In consequence, the substrate spacing can be maintained while greatly suppressing disturbance of the orientation around the resin.
A known liquid crystal electro-optical device using a PCS is shown in FIG. 1. This device comprises transparent substrates 110, 111, electrodes 112, 113, orientation films 114, 115, a liquid crystal material 116, spacers 118, and a sealing material 119. Resinous columns 117 are formed by precipitating (depositing) a resin out of a mixture of the liquid crystal material and an uncured resin and then curing the resin. These resinous columns are bonded to both orientation films 114 and 115 to thereby maintain the substrate spacing. Where an orientation film is formed on only one electrode, one side of each resinous column is bonded to the corresponding electrode or substrate.
Resins shrink greatly in volume on curing. This is a disadvantage with a PCS. Specifically, after a resin and a liquid crystal separate, the molecules of the liquid crystal are oriented without difficulty around uncured resin. As the resin is cured, it shrinks, thus disturbing the orientation. This deteriorates the optical characteristics, especially the characteristic in dark state. Hence, the contrast drops.
These optical characteristics may not stable, i.e., may vary with a lapse of time. One factor representing the stability of these characteristics is a voltage holding ratio. In particular, a voltage is applied to a pixel in a short time and then the application is stopped. The voltage holding ratio is the ratio of the remaining voltage to the applied voltage. In other words, this factor expresses the degree to which the liquid crystal molecule orientation is maintained. Therefore, in order to increase the voltage holding ratio, the liquid crystal molecules should be more stable, and the orientation should be more uniform.
In a liquid crystal electro-optical device using a PCS, when the liquid crystal molecules are urged to be oriented uniformly along the rubbing direction of the orientation film, the molecules might be oriented vertically or randomly, even if the same liquid crystal and the same resin are used.
One factor affecting the orientation of liquid crystal molecules is a force of the orientation film that restricts the liquid crystal molecules. This restricting force of the orientation film attracts the liquid crystal molecules. Simply, this force is given by surface tension, which is affected by various factors such as the kind of orientation film, baking temperature, film thickness, and the structure of the orientation film surface including rubbing.
Also, the surface tension of the PCS (polymerized column spacer) is a major factor which affects the orientation of liquid crystal molecules. The surface tension varies, depending on whether the resin is an oligomer, a monomer, or a combination thereof.
The orientation of the liquid crystal material of a liquid crystal electro-optical device using a PCS is exactly the same as the orientation of a normal liquid crystal display having no resin except around the PCS. However, the liquid crystal molecules around the PCS are restricted not only by the orientation film but also by the PCS. This deteriorates the orientation.
In recent years, large area liquid crystal displays have attracted attention. Where cheap glass substrates or resinous substrates (plastic substrates) are used for large area displays, the substrates themselves are distorted. Furthermore, designing a liquid crystal display operating at a high speed is a matter of great concern and has been studied. To satisfy these requirements, use of a fast-response liquid crystal material such as a ferroelectric liquid crystal may be contemplated.
However, where a ferroelectric liquid crystal is used as the liquid crystal material to accomplish a fast response, it is impossible to fabricate a large area liquid crystal display, for the following reason. A ferroelectric liquid crystal has a layered structure and so if the substrates are deformed, this layered structure is destroyed. This hinders displaying an image. This problem is not restricted to ferroelectric liquid crystals but essentially takes place where various liquid crystal materials are used.
In the past, silicon oxide spacers held between substrates have been used to hold the substrate spacing, and an organic resinous material held between the substrates has been used to maintain the substrates in intimate contact with each other. Literally, spacers are employed to hold the substrate spacing. The substrate spacing is determined by the diameters of the spacers. The organic resin used to maintain substrates in intimate contact with each other has a larger diameter than the required substrate spacing. Organic resin spacers are crushed between the substrates and thus the two substrates are brought into intimate contact with each other.
The prior art configuration described above has been essentially fabricated in the manner described now. The substrate surfaces are first oriented. Then, the above-described spacers and resin are dispersed on one substrate. Subsequently, the substrates are bonded together. In this way, the substrate spacing is determined. Thereafter, a liquid crystal material is injected between the substrates. During this injection, the molecules of the liquid crystal material can be oriented according to the orientation-restricting force of the orientation film, by heating the liquid crystal material, for example at 100.degree. C., and then slowly cooling the material at a rate of 5.degree. C./hr.
We have discussed the above-described manufacturing process and found that the resinous material for maintaining the substrates in intimate contact with each other restricts movement of the liquid crystal molecules when they are being oriented and are changing their state.
In an attempt to solve the above-described two problems, i.e., (1) a structure for maintaining the substrate spacing is needed and (2) when the molecules of the liquid crystal material are oriented, the material for maintaining the substrates in intimate contact with each other adversely affects the orientation of the liquid crystal material, we have devised a method as disclosed in the above-cited Japanese Patent application Ser. No. 55237/1993. In this method, a pair of transparent substrates having electrodes on their surfaces are disposed opposite to each other such that the electrodes are located inside. A liquid crystal material is placed between the substrates. Orientation means is provided on the inner surface of at least one substrate to orient the molecules of the liquid crystal material in a given direction. Uncured resin contained in the liquid crystal material is precipitated (deposited) and cured to form resinous columns. These columns are in intimate contact with the orientation means or with the substrate.
In order to fabricate this liquid crystal electro-optical device, a mixture of the liquid crystal material and a resinous material to which a reaction initiator has been added is sealed in between the transparent opposite substrates whose surfaces have been oriented. The molecules of the liquid crystal material are oriented and then the resinous component is cured by UV irradiation and shaped into columns.
The structure described above is briefly described now by referring to FIG. 1. Shown in this figure is a simple matrix liquid crystal display having transparent substrates 110 and 111 equipped with electrodes 113 and 112, respectively. Orienting means 114 and 115 for orienting the liquid crystal material molecules in given directions are formed on the substrates, respectively. The liquid crystal material, indicated by 116, is sandwiched between the substrates. The molecules of the liquid crystal material 116 are oriented along their respective one axes by the orienting means 114 and 115. A resin 117 is separated and precipitated (deposited) from the liquid crystal material, and is shaped into columns. These resinous columns are bonded to the orienting means 114 and 115 on the substrates 110 and 111, respectively. Where orientation means is provided on only one substrate, the resinous columns 117 are bonded either to the orienting means 114 and the transparent substrate 111 or to the substrate 111 and the electrodes 113.
This liquid crystal electro-optical device is fabricated in the manner described now. A mixture of the liquid crystal material and the uncured resin to which the reaction initiator has been added is held between the transparent substrates 110 and 111 having the electrodes 113 and 112, respectively. The uncured resin is precipitated (deposited) out of the mixture between the substrates to orient the liquid crystal molecules along the orientation means. Then, the precipitated (deposited) resin is cured into the columns 117. These bond together the substrates.
Where the structure shown in FIG. 1 is adopted, the molecules of the liquid crystal material 116 are oriented by the orienting means 115 and then the resin is cured. Therefore, good orientation of the liquid crystal prior to the curing can be maintained. The cured resin little affects the orientation. The resinous columns 117 maintain the substrate spacing. In addition, the columns improve the adhesion of the substrates. Furthermore, the columns enhance the orientation of the liquid crystal molecules.
Preferably, the resinous material used in the above-described structure shows a mixture state with the liquid crystal material at high temperatures and is separated and precipitated (deposited) from the liquid crystal material in low temperatures.
It is very desired that the uncured resin contain no solvent in order to cure the resin when it is held between the substrates. Since the separation into the liquid crystal material and the resin and the orientation of the liquid crystal material depend heavily on temperature, it is desired to cure the resin independent of temperature. Considering these factors into consideration, we can say that using a UV curable resin as the uncured resin and ultraviolet radiation as the resin-curing means is desired. In the above-described configuration, limitations are imposed neither on the kind of the liquid crystal material nor on the operating mode of the liquid crystal.
Where liquid crystal molecules are oriented parallel to the substrates in operation, it is necessary that these liquid crystal molecules obey the orientation-restricting force. However, where the resinous component is precipitated (deposited) into columns in the liquid crystal, the liquid crystal molecules are observed to be oriented parallel to, and along, the resinous columns around these columns. This means that the long axes of some liquid crystal molecules are oriented vertical to the substrates, although the long axes of all liquid crystal molecules must be oriented parallel to the substrates. If the disturbance of the orientation becomes conspicuous, the optical characteristics, especially the dark state characteristic, deteriorate. This leads to a decrease in the contrast. That is, where the resinous component is precipitated (deposited) into columns out of the liquid crystal mixture, the contrast is deteriorated in practice.